Hi,
Not sure is this new thing or havent this worked with me before.
I use my Android to control my Sonos system trough Spotify app. For my phone I normally use my same wifi as my Sonos system has. Now when I use my mobiledata 4G in my phone I can control my Sonos system (that is still connected to the wifi!) normally with no problems.
Has it always been like this as using Sonos S2 app it demands to be in the same network as the Sonos System but in Spotify this is not the case.